ASHRAE Journal - February 2021 - 75

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Procedure, the 1981 standard included a list of 20 compounds or classes of compounds, with concentration
limits for ﬁve of them: CO2, chlordane, formaldehyde,
ozone and radon. All of the limits were linked to a U.S. or
other national government reference with the exception
of CO2. The CO2 limit of 4,500 mg/m3 is discussed in an
appendix to the 1981 standard, which noted (without
reference) that 0.5% CO2 is a good limit based on concerns about headaches and loss of judgment. A safety
factor of two is then used to account for variations in
individual activity, diet and health, leading to the stated
limit of 0.25% (about 4,500 mg/m3). The 1989 standard
contained concentration limits for four contaminants
(CO2, chlordane, ozone and radon) for use with the
IAQ Procedure. The CO2 limit in the 1989 standard was
1,800 mg/m3 (roughly 1,000 ppmv), 60% lower than the
value in the 1981 standard, but no explanation was provided for this reduction.

assumed values of CO2 generation, 7.5 L/s (15 cfm) will
lead to a steady-state CO2 concentration that is 700 ppmv
above outdoors. That explanation, which is not a CO2
concentration limit, remained in the standard through
2016 and was removed from the 2019 standard.
Understand that the relationship of 7.5 L/s (15 cfm)
and 1,000 ppmv is only relevant to spaces for which
7.5 L/s (15 cfm) is the outdoor air ventilation requirement.
While ofﬁce spaces are required to provide about 7.5 L/s
(15 cfm) per person (depending on occupant density),
other spaces have ventilation requirements ranging from
less than 3 L/s to 12 L/s (6 cfm to 25 cfm) or more. In those
cases, the steady-state CO2 concentration will be quite different from 1,000 ppmv, ranging from roughly 700 ppmv
to 5,000 ppmv, again depending on the occupant density.
Therefore, identifying relevant CO2 concentrations that
correspond to ventilation rate requirements must consider the building type and its occupancy.

More Recent Versions of Standard 62.1

Conclusions

Subsequent versions of Standard 62 in 1999 and 2001
retained the contaminant limits that were in the 1989
standard, although CO2 was removed from the table in
1999. That table was removed entirely from the 2004
version of the standard, with all discussions of contaminant limits contained only in informative appendices
(not ofﬁcially part of the standard).
The confusion regarding CO2 in Standard 62.1 is likely
associated with an informative appendix that was
added in 1989. That appendix explained the connection between per person outdoor air ventilation rates
and steady-state levels of CO2. That discussion notes
that for speciﬁed values of CO2 generation by a person
and the outdoor CO2 concentration, a ventilation rate
of 7.5 L/s (15 cfm) per person will lead to a steady-state
CO2 concentration of 1,000 ppmv. That discussion was
apparently interpreted by some as justifying the 1,000
ppmv limit in the body of the standard under the IAQ
Procedure, but that is not what the standard stated.
As noted above, the 1,000 ppmv limit was removed
from the standard in 1999, and the informative appendix was modiﬁed to better explain the connection
between CO2 concentrations and bioefﬂuent perception. The modiﬁed appendix noted that 7.5 L/s (15 cfm)
of outdoor air will dilute bioefﬂuent odors such that
about 80% of unadapted persons (visitors) are satisﬁed
in their perception of those odors. It again noted that for

Despite the fact that ASHRAE Standard 62.1 has not
contained an indoor CO2 concentration limit for the
past 30 years, there are many instances in which practitioners and researchers make claims that a building
has good IAQ because it complies with the 1,000 ppmv
CO2 limit in the standard. More recent versions of the
standard do not include any statement implying that
1,000 ppmv is a guideline or target value. While the
direct impacts of indoor CO2 concentrations on human
health, comfort and performance are of interest, and
new research is being conducted to examine those
impacts, there is not yet sufﬁcient justiﬁcation to change
existing ventilation standards.4 Additional information
on the history of Standard 62, including the role of CO2,
is found in Reference 5.

References

1. ANSI/ASHRAE Standard 62.1-2019, Ventilation for Acceptable
Indoor Air Quality.
2. Persily, A. 1997. " Evaluating building IAQ and ventilation with
indoor carbon dioxide. " ASHRAE Transactions 103(2):193 - 204.
3. ASTM. 2018. " ASTM D6245-2018, Standard Guide for Using
Indoor Carbon Dioxide Concentrations to Evaluate Indoor Air
Quality and Ventilation. " ASTM International.
4. Fisk, W., P. Wargocki, X. Zhang. 2019. " Do
indoor CO2 levels directly affect perceived air
quality, health, or work performance? " ASHRAE
Journal 61(9):70 - 77.
5. Persily, A. 2015. " Challenges in developing https://bit.ly/3aC5wVH
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ventilation and indoor air quality standards: The
story of ASHRAE Standard 62. " Building and Environment 91:61 - 69.
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